The treatments involve exposing the skin to ultraviolet light. The exposures can be to small area of the skin or over the whole body surface, like in a tanning bed. The most common treatment is with narrowband UVB (NB-UVB) with a wavelength of 311-313 nanometer. It was found that this is the safest treatment[1] Full body phototherapy can be delivered at doctor's office or at home using a large high power UVB booth.[2]

For psoriasis, UVB phototherapy has been shown to be effective.[3] A feature of psoriasis is localized inflammation mediated by the immune system.[4]Ultraviolet radiation is known to suppress the immune system and reduce inflammatory responses. Light therapy for skin conditions like psoriasis usually use NB-UVB (311 nm wavelength) though it may use UV-A (315–400 nm wavelength) or UV-B (280–315 nm wavelength) light waves. UV-A, combined with psoralen, a drug taken orally, is known as PUVA treatment. In UVB phototherapy the exposure time is very short, seconds to minutes depending on intensity of lamps and the person's skin pigment and sensitivity. The time is controlled with a timer that turns off the lamps after the treatment time ends. William H. Goeckerman pioneered

One percent of the population suffer from vitiligo, and narrowband UVB phototherapy is an effective treatment. "NB-UVB phototherapy results in satisfactory repigmentation in our vitiligo patients and should be offered as a treatment option."[5]

Evidence for light therapy and lasers in acne vulgaris as of 2012 is not sufficient to recommend them.[6] There is moderate evidence for the efficacy of blue and blue-red light therapies in treating mild acne, but most studies are of low quality.[7][8] While light therapy appears to provide short term benefit, there is a lack of long term outcome data or data in those with severe acne.[9]

According to the American Cancer Society, there is some evidence that ultraviolet light therapy may be effective in helping treat certain kinds of skin cancer, and ultraviolet blood irradiation therapy is established for this application. However, alternative uses of light for cancer treatment – light box therapy and colored light therapy – are not supported by evidence.[10] Photodynamic therapy (often with red light) is used to treat certain superficial non-melanoma skin cancers.[11]

Low level laser therapy has been studied as a potential treatment for chronic wounds.[14] Reviews of the scientific literature do not support the widespread use of this technique due to inconsistent results and low research quality.[14][15] Higher power lasers have also been used to close acute wounds as an alternative to stitching.[14]

Full sunlight or exposure to bright light from a light box is used to treat seasonal affective disorder (SAD). Light boxes for SAD are designed to filter out most UV light, which can cause eye and skin damage.[18]Mayo Clinic states that light therapy is a proven treatment for seasonal affective disorder.[19] It is considered a first-line treatment.[20] Controlled-trial comparisons with antidepressants show equal effectiveness, with less expense and more rapid onset of therapeutic benefit, though a minority of patients may not respond to it.[21] Direct sunlight, reflected into the windows of a home or office by a computer-controlled mirror device called a heliostat, has also been used as a type of light therapy for the treatment of SAD.[22][23]

The effectiveness of light therapy for treating SAD may be linked to the fact that light therapy makes up for lost sunlight exposure and resets the body's internal clock.[24][25] Studies show that light therapy helps reduce the debilitating and depressive behaviors of SAD, such as excessive sleepiness and fatigue, with results lasting for at least 1 month. Light therapy is preferred over antidepressants in the treatment of SAD because it is a relatively safe and easy therapy.[26]

It is possible that response to light therapy for SAD could be season dependent.[27] Morning therapy has provided the best results because light in the early morning aids in regulating the circadian rhythm.[26]

Light therapy has also been suggested in the treatment of non-seasonal depression and other psychiatric disturbances, including major depressive disorder,[28]bipolar disorder and postpartum depression.[29][30] A meta-analysis by the Cochrane Collaboration concluded that "for patients suffering from non-seasonal depression, light therapy offers modest though promising antidepressive efficacy."[31] A 2008 systematic review concluded that "overall, bright light therapy is an excellent candidate for inclusion into the therapeutic inventory available for the treatment of nonseasonal depression today, as adjuvant therapy to antidepressant medication, or eventually as stand-alone treatment for specific subgroups of depressed patients."[20] A 2015 review found that supporting evidence for light therapy was weak due to serious methodological flaws.[32] However, a 2016 randomized double blind clinical trial (which used a sham light treatment) found that light therapy in combination with Prozac for 8 weeks as well as light therapy alone resulted in a significantly superior decrease in depression scores than placebo.[33]

Photodynamic therapy is a form of phototherapy using nontoxic light-sensitive compounds that are exposed selectively to light, whereupon they become toxic to targeted malignant and other diseased cells

Light therapy uses either a light box which emits up to 10,000 lux of light at a specified distance, much brighter than a customary lamp, or a lower intensity of specific wavelengths of light from the blue (460 nm) to the green (525 nm) areas of the visible spectrum.[41] A 1995 study showed that green light therapy at doses of 350 lux produces melatonin suppression and phase shifts equivalent to 10,000 lux white light therapy,[42][43] but another study published in May 2010 suggests that the blue light often used for SAD treatment should perhaps be replaced by green or white illumination, because of a possible involvement of the cones in melatonin suppression.[44]

In treatment, the patient's eyes are to be at a prescribed distance from the light source with the light striking the (lower) retina. This does not require looking directly into the light.

Considering three major factors – clinical efficacy, ocular and dermatologic safety, and visual comfort, the Center for Environmental Therapeutics (CET) recommends the following criteria for light box selection:[45]

Light boxes should have been tested successfully in peer-reviewed clinical trials.

The box should provide 10,000 lux of illumination at a comfortable sitting distance. Product specifications are often missing or unverified; illuminance can be controlled using a light meter.

Fluorescent lamps should have a smooth diffusing screen that filters out ultraviolet (UV) rays. UV rays are harmful to the eyes and skin.

Blue light is known to be superior to red light in managing depressive symptoms which have a seasonal pattern.[46]

The light should be projected downward toward the eyes at an angle to minimize aversive visual glare.

Smaller is not better; when using a compact light box, even small head movements will take the eyes out of the therapeutic range of the light.

Modern phototherapy lamps used in the treatment of seasonal affective disorder and sleep disorders either filter out or do not emit ultraviolet light and are considered safe and effective for the intended purpose, as long as photosensitizing drugs are not being taken at the same time and in the absence of any existing eye conditions. Light therapy is a mood altering treatment, and just as with drug treatments, there is a possibility of triggering a manic state from a depressive state, causing anxiety and other side effects. While these side effects are usually controllable, it is recommended that patients undertake light therapy under the supervision of an experienced clinician, rather than attempting to self-medicate.[58]

Contraindications to light therapy for seasonal affective disorder include conditions that might render the eyes more vulnerable to phototoxicity, tendency toward mania, photosensitive skin conditions, or use of a photosensitizing herb (such as St. John's wort) or medication.[59][60] Patients with porphyria should avoid most forms of light therapy. Patients on certain drugs such as methotrexate or chloroquine should use caution with light therapy as there is a chance that these drugs could cause porphyria.[citation needed]

Side effects of light therapy for sleep phase disorders include jumpiness or jitteriness, headache, eye irritation and nausea.[61] Some non-depressive physical complaints, such as poor vision and skin rash or irritation, may improve with light therapy.[62]

Many ancient cultures practiced various forms of heliotherapy, including people of Ancient Greece, Ancient Egypt, and Ancient Rome.[63] The Inca, Assyrian and early German settlers also worshipped the sun as a health bringing deity. Indian medical literature dating to 1500 BCE describes a treatment combining herbs with natural sunlight to treat non-pigmented skin areas. Buddhist literature from about 200 CE and 10th-century Chinese documents make similar references.

The FaroesephysicianNiels Finsen is believed to be the father of modern phototherapy. He developed the first artificial light source for this purpose.[citation needed] Finsen used short wavelength light to treat lupus vulgaris, a skin infection caused by Mycobacterium tuberculosis. He thought that the beneficial effect was due to ultraviolet light killing the bacteria, but recent studies showed that his lens and filter system did not allow such short wavelengths to pass through, leading instead to the conclusion that light of approximately 400 nanometers generated reactive oxygen that would kill the bacteria.[64] Finsen also used red light to treat smallpox lesions. He received the Nobel Prize in Physiology or Medicine in 1903.[65] Scientific evidence for some of his treatments is lacking, and later eradication of smallpox and development of antibiotics for tuberculosis rendered light therapy obsolete for these diseases.[66]